1. Field of the Invention
This invention generally relates to a vacuum interrupter, and more particularly to an axial magnetic field applied type vacuum interrupter which applies an axial magnetic field in parallel to an arc current path produced between separated electrodes within the vacuum envelope of the interrupter.
2. Description of the Prior Art
U.S. Pat. No. 4,661,666 issued Apr. 28, 1987 discloses a prior-art vacuum interrupter as shown in FIG. 1. This interrupter has a vacuum envelope 1 and a disc-shaped stationary electrode 2 and a movable electrode 3 disposed within the vacuum envelope 1 and operable for forming or interrupting electrical contact therebetween. The vacuum envelope 1 comprises an insulating cylinder 4, a disc-shaped metal end plate 5 hermetically secured to one edge of the insulating cylinder 4 via a metal seal ring 6, a bottomed metal cylinder 7 the open end of which is hermetically secured to the other edge of the insulating cylinder 4 via a metal seal ring 6. The stationary and movable electrodes 2 and 3 are located within the metal cylinder 7.
A stationary lead rod 9 passes hermetically through and is fixed to a flat bottom 7a of the metal cylinder 7. An inner end of the stationary lead rod 9 carries the stationary electrode 2 within the metal cylinder 7. On the other hand, a movable lead rod 10 passes loosely through the metal end plate 5 and is hermetically secured to the metal end plate 5 via a metal bellows 11. An inner end of the movable lead rod 10 carries the movable electrode 3 within the metal cylinder 7. Thus, the movable lead rod 10 is considerably longer than the stationary lead rod 9. The bellows 11 is located within the insulating cylinder 4 with its inner surface exposed to the atmosphere. The bellows 11 is as remote from the electrodes 2 and 3 within the vacuum envelope 1 as possible in order to protect the bellows 11 from the deposition of the metal vapor generated by the electrodes 2 and 3 during opening and closing operations. A cup-shaped bellows shield 12 is fixed to an intermediate portion of the movable lead rod 10. The bellows shield 12 also protects an inner end area of the bellows 11 from deposition of the metal vapor.
A coil 13 of substantially one turn surrounds the stationary and movable electrodes 2 and 3 outside the cylindrical portion of the metal cylinder 7. The coil 13 produces an axial magnetic field running parallel to the arc current path between the separated stationary and movable electrodes 2 and 3 for dispersing the arc evenly across the opposing faces of the electrodes thereby increasing the current interruption performance of the interrupter. One end 13a of the coil 13 is electrically connected to an outer end of the stationary lead rod 9. The other end 13b of the coil 13 is electrically connected to one end of an outer lead rod 14 which is located outside the vacuum envelope 1. The outer lead rod 14 extends perpendicularly to the stationary lead rod 9.
An outer lead rod 15 which is located outside the vacuum envelope 1 extends parallel to the outer lead rod 14. One end of the outer lead rod 15 has a slide contact 16 which mechanically and electrically engages an outer end of the movable lead rod 10. A main shield 17 is fixed to an inner cylindrical surface of the metal cylinder 7. The electrical potential of the main shield 17 is equal to that of the stationary lead rod 9 but different from that of the movable lead rod 10. An auxiliary shield 18 is fixed to the end plate 5.
In the operation of the above-described interrupter, a current (e.g., a fault current) passes through a sequence comprising the outer lead rod 14, the coil 13, the stationary lead rod 9, the stationary electrode 2, the arc current path between the stationary electrode 2 and the movable electrode 3, the movable electrode 3, the movable lead rod 10, the slide contact 15 and the outer lead rod 15 and vice versa. Therefore, the stationary and movable lead rods 9 and 10 are subjected to a resulting electro-magnetic force with a radial vector in accordance with the left-hand rule when a current passes through the above-described sequence. The electro-magnetic force radially inclines the movable lead rod 10 when the stationary and movable electrodes 2 and 3 are out of contact. This inclination displacement reduces the clearance between the movable lead rod 10 and the main shield 17 which have different potentials, which in turn reduces the dielectric strength of the vacuum interrupter. An inclination displacement of the movable lead rod 10 due to the electro-magnetic force of the coil 13 causes the stationary and movable electrodes 2 and 3 to be in point-to-point contact at outer peripheries of the stationary and movable electrodes 2 and 3. Thus, a mechanical impact force occurring during closing operation of the stationary and movable electrodes 2 and 3 concentrates at the point of contact between the stationary and movable electrodes 2 and 3. This concentration of the mechanical impact force can possibly split or break the stationary and movable electrodes 2 and 3 during many opening and closing operations. Thus the radial displacement of the movable electrode 2 causes premature wear and reduced dielectric strength in the vacuum interrupter. Furthermore, the lengthiness of the movable lead rod 10 increases the total weight of the movable assembly associated with the movable lead rod 10, and the load of weight on the associated operating mechanism for the movable lead rod 10.
Most of the metal vapor produced during the opening operation of the stationary and movable electrodes 2 and 3 disperses to a space behind the movable electrode 3 in the insulating cylinder 4 rather than the space behind the stationary electrode 2 because the space behind the movable electrode 3 is greater than the space behind the stationary electrode 2. Therefore, some of the dispersing metal vapor deposits on the surface of the bellows 11 during many (no less than 10,000 times) opening and closing operations in spite of the presence of the bellows shield 12. The metal vapor deposited on the bellows 11 melts a little bit of the surface of the bellows 11 and causes the adjacent annular portions of the bellows 11 to stick each other because the bellows 11 contracts during the opening operation of the stationary and movable electrodes 2 and 3 when the vapor is formed. The sticking together of the adjacent annular portions of the bellows causes them to tear and leak thus compromizing the vacuum within the vacuum envelope 1.
In the prior-art vacuum interrupter, the short stationary lead rod 9 connects the stationary and movable electrodes 2 and 3 to the coil 13, so that Joule heat due to contact resistance between the stationary and movable electrodes 2 and 3 cannot be dissipated sufficiently through the stationary lead rod 9. Moreover, Joule heat produced by the coil 13 is added to that produced by contact resistance. Thus, the temperature of the vacuum interrupter may be caused to exceed the maximum temperature (e.g., a temperature of a silver-plating-free lead rod being 90.degree. C. under an ambient temperature of 40.degree. C.) permissible for the vacuum interrupter.
In addition, the vacuum interrupter usually constitutes part of a circuit breaker installed in a metal-clad switchgear, the stationary lead rod 9 being located in an upper portion of the vacuum interrupter. Thus, the coil 13 as a heat transmitter surrounds the upper portion of the vacuum interrupter. This arrangement blocks the natural convection along the outer length of the vacuum envelope within the surrounding atmosphere, thus blocking heat dissipation from the vacuum interrupter.